In allergic and non-allergic asthma, eosinophils are prominent in both blood and sputum, and evidence now strongly suggests that the eosinophil is a critical cell in the pathophysiology of asthma and possibly its pathogenesis. We previously proposed that activated eosinophils circulate in the blood of patients with asthma and that these activated eosinophils release toxic oxygen products which have the capacity to produce tissue injury and may reflect similar activity in airways. Characterization of eosinophil function and its contribution to asthma have been advanced by newer methods of cell isolation. Consequently, it is now known that human eosinophils are heterogeneous in density, surface receptors, and metabolic function. Moreover, the hypodense eosinophil, whose presence is increased in asthma, has enhanced functional activity and hence greater capacity for tissue injury. We now propose to extend our previous observations by examining the hypothesis that the hypodense eosinophil in asthma has an increased capacity to generate toxic oxygen products which can contribute to airway injury. To accomplish this, we will further refine our isolation procedure to purify hypodense eosinophils from blood and bronchoalveolar lavage (BAL) fluid. Functional activity of the normodense and hypodense eosinophils will be compared by measurement of chemiluminescence, superoxide generation, H2O2 secretion and degranulation; levels of eosinophil activity will be correlated to asthma severity and airway hyperractivity; and the response of an in vivo antigen and methacholine bronchial challenge on peripheral blood eosinophil density and function will be determined. Parallel studies, including indirect immunofluorescence with the fluorescent activated cell sorter, will identify surface receptors for immunoglobulins and complement. Finally monoclonal antibodies will be developed to the hypodense eosinophil to determine whether unique surface markers exist on this cell and participate in eosinophil function. We anticipate these results to be important to establish not only the contribution of eosinophils to asthma but also a better understanding of this cell's metabolic and immune function.